Lithium secondary batteries are small and light weight and have high energy-density and excellent storage characteristics. For these advantages, lithium secondary batteries have been widely used as main and backup power sources of various electronic appliances. In general, a lithium secondary battery used to power an electronic appliance has the shape of a coin or cylinder and mounted directly on a circuit board, such as a printed circuit board. In order to mount a lithium secondary battery onto a circuit board, one or more metal lead plates are secured at a first end portion to one or more of the electrode terminals of the lithium secondary battery by spot welding or laser welding. In addition, a second end portion of each lead plate is secured to the circuit board by soldering, after the second end portion is inserted into a terminal hole provided through the circuit board or placed onto the circuit board. (Note that the terms “lead” and “lead plate” are interchangeably used in this specification.)
One common process employed to reduce the cost of mounting electronic components onto a circuit board is reflow soldering, which is a process of placing electronic components on a surface of a circuit board and attaching the electronic components to the circuit board by reflowing solder paste. Typically, the solder reflowing process includes coating a surface of the circuit board with solder cream, positioning electronic components onto the solder cream coating, and applying heat by transporting the entire circuit board into a conveyorized reflow furnace to melt the solder cream. Taking advantage of the above process, the reflow soldering is also used to automatically solder a second end portion of a metal lead plate to a circuit board, after a first end portion of the metal lead plate has been attached to an end face of an electrode.
In order to realize such soldering, solder cream is applied to appropriate surface areas of a circuit board and the second end portion of each lead plate is positioned onto the solder cream coatings. Thereafter, the lithium secondary battery and the circuit board are transported into a reflow furnace where heat of 230° C.-270° C. is applied for a short period of time to melt the solder cream. When the molten solder solidifies, the lithium secondary battery is securely joined to the circuit board.
Unfortunately, however, the following problems should be noted. FIGS. 6A and 6B show a coin battery 20X to which leads have been soldered to the circuit board by the reflow soldering described above. As shown in the figures, a coin battery 10 has an anode cap 12 acting as one of the electrode terminals and a cathode can 11 acting as the other electrode terminal. A lead plate 21 is welded at a first end portion to the anode cap 12, whereas a lead plate 22 is welded at a first end portion to the cathode can 11. The second end portions of the lead plates 21 and 22 constitute soldering potions 21a and 22a. Through melting of solder cream coatings 21b and 22b applied to conductive portions U1 and U2 of a circuit board U, the soldering portions 21a and 22b are joined to the circuit board U. As a result, the coin battery 20X with leads is mounted on the circuit board U.
The problem is, as shown in FIG. 6B, that the soldering potions 21a and 22a of the lead plates 21 and 22 project outwardly beyond the peripheral edge of the coin buttery 10. Because of the soldering portion 21a of the lead plate 21 and the soldering portion 22a of the lead plate 22, the mounting area of the coin battery 10 on the circuit board U inevitably increases. In order to more firmly bond the coin battery 10 to the circuit board U, the area of the soldering portions 21a and 22a need to be increased even further, which results in increase of the mounting area even further.
Patent Document 1 (JP 2002-298804-A) suggests the following coin batteries each provided with leads, without increasing the area of soldering portions. FIG. 7A shows a coin battery 20Y to which leads have been attached according to Patent Document 1. As shown in the figure, the coin battery 10 has the cathode can 11 acting as one of the electrode terminals and the anode cap 12 acting as the other electrode terminal. A first end portion of a lead plate 23 is welded to the cathode can 11. A second end portion of the lead plate 23 is soldered to a circuit board V by melting a solder cream coating 23b applied to a conductive portion V1 of the circuit board V. The anode cap 12, however, is soldered to the circuit board V directly without any lead plate, by melting a solder cream coating 24 applied to a conductive portion V2 of the circuit board V.
FIG. 7B shows another coin battery 20Z to which leads have been attached according to Patent Document 1. As shown in the figure, a coin battery 10 has the cathode can 11 acting as one of the electrode terminals. A first end portion of a lead plate 25 is welded to the cathode can 11. The lead plate 25 is bent so that a second end portion thereof does not project outwardly beyond the peripheral edge of the cathode can 11. The second end portion of the lead plate 25 is soldered to a circuit board W by melting a solder cream coating 25b applied to a conductive portion W1 of the circuit board W. In addition, the anode cap 12 doubling as the other electrode of the coin battery 10 is soldered directly to the circuit board W without any lead plate, by melting a solder cream coating 26 applied to a conductive portion W2 of the circuit board W. With this arrangement, the coin battery 10 is mounted to the circuit board W without increasing the mounting area too much.    Patent Document 1: JP 2002-298804